Chantal Hamel

Acquisition of Cu, Zn, Mn and Fe by mycorrhizal maize (Zea mays L.) grown in soil at different P and micronutrient levels

Abstract Sustainability of soil-plant systems requires, among other things, good development and function of mycorrhizal symbioses. The effects of P and micronutrient levels on development of an arbuscular mycorrhizal fungus (AMF) and uptake of Zn, Cu, Mn and Fe by maize (Zeamays L.) were studied. A pot experiment with maize either inoculated or not with Glomus intraradices was conducted in a sand:soil (3 :1) mix (pH 6.5) in a greenhouse. Our goal was to evaluate the contribution of mycorrhizae to uptake of Cu, Zn, Mn and Fe by maize as influenced by soil P and micronutrient levels. Two levels of P (10 and 40 mg kg−1 soil) and three levels of a micronutrient mixture: 0, 1X and 2X (1X contained, in mg kg−1 soil, 4.2 Fe, 1.2 Mn, 0.24 Zn, 0.06 Cu, 0.78 B and 0.036 Mo), were applied to pots. There were more extraradical hyphae at the low P level than at the high P level when no micronutrients were added to the soil. Root inoculation with mycorrhiza and application of micronutrients increased shoot biomass. Total Zn content in shoots was higher in mycorrhizal than non-mycorrhizal plants grown in soils with low P and low or no micronutrient addition. Total Cu content in shoots was increased by mycorrhizal colonization when no micronutrients were added. Mycorrhizal plants had lower Mn contents than non-mycorrhizal plants only at the highest soil micronutrient level. AMF increased total shoot Fe content when no micronutrients were added, but decreased shoot Fe when plants were grown at the high level of micronutrient addition. The effects of G. intraradices on Zn, Cu, Mn, and Fe uptake varied with micronutrient and P levels added to soil.

Dynamics of the mycorrhizal symbiosis of corn (Zea mays L.): effects of host physiology, tillage practice and fertilization on spatial distribution of extra-radical mycorrhizal hyphae in the field

Abstract Tillage and fertilization may reduce the abundance of indigenous arbuscular mycorrhizal (AM) fungi in agricultural field soils. The dynamics of hyphal abundance in soil were studied over two growing seasons at a site in eastern Canada in a corn crop grown in a sandy loam soil and over one growing season in a corn crop grown in a clay soil. Experimental plots in a long-term tillage experiment, had been managed under no-tillage (NT), reduced tillage (RT) and conventional tillage (CT) for 11 years. Soil receiving each of these tillage treatments also received either inorganic (N and K) or organic (liquid dairy manure) fertilizer. Soil samples were collected from different places within each plot: on the plant row, at 18.75 cm from the row (quarter of the inter-row distance) and in between two rows (mid-row, i.e., 37.5 cm from the adjacent rows). Plant and soil samples were taken at the 12–14 leaf stage of corn (June), at silking stage (August) and at harvest (October), in order to measure the fluctuation in soil hyphal densities and plant nutrients concentrations during the season. Densities of total and viable AM hyphae were greatest in the row and lowest in the mid-row. Hyphal density on the row increased steeply from 12–14 leaf stage to silking stage and decreased thereafter. No significant fluctuation of hyphal abundance was observed in the mid-row, suggesting a prevalence of AM hyphae on the row. Hyphal densities were higher in NT soil than in CT soil, while RT soil contained intermediate hyphal densities. The highest corn P, Zn and Cu concentrations were observed in NT and RT treatments concurrently with the highest hyphal densities. Concentrations of K, Ca and Mg did not change with tillage or fertilization type. Manure application significantly increased the densities of total and viable hyphae in the clay soil.

Vertical distribution of arbuscular mycorrhizal fungi under corn (Zea mays L.) in no-till and conventional tillage systems

Abstract We compared the vertical distribution (0–25 cm) of arbuscular mycorrhizae, extraradical hyphae, and glomalean spores at grain-filling of corn under conventional tillage versus no tillage. Root colonization, total hyphae density, and spore density were correlated, and were highest at a depth of 0–15 cm in soil. Tillage significantly reduced total hypha density and spore density at 0–5 cm depth, but did not affect root colonization. Plowing below 15 cm is likely to diminish AM fungus inocula in the rooting zone of establishing seedlings.

Prospects and problems pertaining to the management of arbuscular mycorrhizae in agriculture

The arbuscular mycorrhizal symbiosis is recognized for its multiple positive effects on plant growth and for its important contribution towards the maintenance of soil quality. In spite of these benefits to agriculture, at present, the realization of the full potential of this symbiosis has not yet been reached. The understanding of interactions existing among crops, fungal partners and environmental conditions must improve to allow for the efficient management of the mycorrhizal symbiosis through selected agronomic practices and inoculation of cultivated crops.

Plant development in a mycorrhizal field-grown mixture

Abstract In the field, a mycorrhizal mixture of corn and soybean was compared to non-mycorrhizal and to P-compensated plant mixtures. The extent of 15N-transfer from soybean to corn was assessed. Plant development and the competitive relationship between the components of the mixtures were also examined. After having labelled selected soybean plants with isotoplc NH4NO3 by feeding roots induced on their stems, a greater amount of 15N-transfer to corn was measured in mycorrhiza inoculated plots than in control plots. The growth of both corn and soybean plants was greatly enhanced when inoculated with Glomus intraradix, and the effect of the fungus could not be replicated by fertilization. Inoculation and P fertilization had similar effects on P, K and Mg uptake by plants, but their effects differed regarding Ca absorption. Inoculation with the mycorrhizal fungus favoured the grass component of the mixture over the legume. Even if more N appeared to be transferred from soybean to corn when plants were mycorrhizal, the nutrient status of the plants suggests that the growth increase can be attributed mainly to a better P uptake by mycorrhizal plants, and that the significance of interspecific mycorrhizae-mediated N-transfer may be limited.

Combined effects of soil disturbance and fallowing on plant and fungal components of mycorrhizal corn (Zea mays L.)

Abstract Soil disturbance may reduce the effect of mycorrhizae on plant growth and nutrient uptake through its effects on the integrity of the extraradical hyphal network. A growth-chamber experiment was conducted to evaluate the survival of extraradical arbuscular mycorrhizal (AM) fungal hyphae when detached from the host root system, and to understand the effects of soil disturbance on the ability of these hyphae to colonize plant roots and to reestablish mycorrhizal associations in previously disturbed soils. The experiment consisted of establishing AM fungi in pots divided into two compartments by a nylon mesh (37 μm), by growing corn (Zea mays L.) in one of the compartments for 6 weeks in an unsterilized agricultural field soil. The mesh prevented the growth of corn roots from one side of the pot to the other, while allowing the passage of the AM hyphae. After establishment of AM fungi the following treatments were performed: soil in the two compartments was either disturbed by sieving through 2 mm mesh (D) or undisturbed (U) leading to four combined disturbance treatments: (1) both compartments undisturbed (UU); (2) root compartment disturbed and root-free compartment undisturbed (DU); (3) root compartment undisturbed and root-free compartment disturbed (UD); and (4) both compartments disturbed (DD). The effects of fallows of four different durations; 0, 30, 60 and 90 d were also measured in the same experiment giving a total of 16 treatments. Soils were disturbed at the beginning of the experiment in the root compartment, and after each fallow period in the root-free compartment. Immediately after disturbance of the soil in the root-free compartment, corn was planted and grown for 30 d to test the combined effects of fallow and soil disturbance on AM formation and nutrient content. Soil disturbance had no adverse effect on AM efficiency if test plants were planted immediately after disturbing the root-free compartment. However, AM efficiency decreased with increasing length of fallow. Lengths of total and metabolically active extraradical hyphae in the root-free compartments were measured before each fallow. Significantly less hyphal lengths were observed in pots where the soil of the root compartment had been disturbed. Test plant shoot weight was highest in UU and lowest in DD treated pots. Phosphorus content by the test plants was twice as high in UU as in DD. Test plants in undisturbed (UU) pots had greater Zn and Cu contents than in DU, UD or DD pots. Content of P, Zn and Cu in test plants was reduced by about 40%, 63% and 70%, respectively, by 90 d of fallow, compared to 0 d fallow.

Indigenous populations of arbuscular mycorrhizal fungi and soil aggregate stability are major determinants of leek (Allium porrum L.) response to inoculation with Glomus intraradices Schenck & Smith or Glomus versiforme (Karsten) Berch

Abstract Knowledge of physical, chemical and biological soil characteristics influencing plant response to inoculation with arbuscular mycorrhizal (AM) fungi would help to distinguish soils where inoculation could be profitable. The relationship between leek (Allium porrum L.) response to mycorrhizal inoculation with Glomus intraradices Schenck & Smith or G. versiforme (Karsten) Berch and soil texture, bulk density, particle density, porosity, pH, organic matter content, available P, K, Ca, Mg, Fe, Zn, Cu, and Mn, soil structure, soil mycorrhizal potential (SM), preceding crop mycorrhizal potential, composition of indigenous mycorrhizal fungal communities, and the abundance of spores of different species, was studied in 81 agricultural soils using Principal Component Analysis and regression analysis. The nature of the indigenous AM fungi population was an important determinant of leek response to inoculation (RTI). In soils with more than 200 μg available P g–1, SM potential accounted for over 27% of RTI with G. intraradices and G. versiforme, RTI being high in soils with low SM potential. In low P soils, however, a positive relation between the abundance of water stable soil aggregates in the 0.5–2 mm diameter range and RTI was most important. Low soil Zn and high porosity, abundant total mycorrhizal spore as well as scarcity of spores of G. aggregatum and of the group G. etunicatum-rubiforme were also associated to high RTI. The influence of water stable aggregation of soil on RTI was modulated by soil P levels. Abundance of soil aggregates was positively related to RTI at low soil P levels, but negatively related to RTI at high P levels. Different relationships were found between soil variables and spore abundance of different AM fungi species. Some AM species appear to have as yet undefined similarities or complementarities at the biological or ecological levels.

Overwinter survival of arbuscular mycorrhizal hyphae is favored by attachment to roots but diminished by disturbance

Abstract We investigated the overwinter survival in the field of indigenous arbuscular mycorrhizal (AM) hyphae either connected to corn roots or detached from them, and either intact or disrupted. We buried soil-filled pouches which either allowed root entry or excluded roots in the root zone of a field-grown corn (Zea mays) crop in eastern Canada. Following crop harvest in the fall, pouches either remained undisturbed, were disturbed outside the pouch, or were disturbed both inside and outside the pouch. Total and metabolically active AM hyphae in undisturbed pouches declined 20% and 33% (average of coarse- and fine-mesh treatments), respectively, from fall to spring, presumably because of death overwinter. In the spring, living hyphae were more abundant in the presence of roots than in their absence, suggesting that attachment or proximity to roots favored overwinter survival. Total hyphal density, metabolically active hyphal density, and the proportion of total living hyphae progressively diminished with increased disturbance.

ROOT-ZONE TEMPERATURE AND SOYBEAN (GLYCINE MAX. (L.) MERR.) VESICULAR-ARBUSCULAR MYCORRHIZAE: DEVELOPMENT AND INTERACTIONS WITH THE NITROGEN FIXING SYMBIOSIS

Abstract Suboptimal root-zone temperature (RZT) has been shown to decrease soybean [Glycine max (L.) Merr.] nodulation and nitrogen fixation. However, there are few studies on the effects of suboptimal RZTs on vesicular-arbuscular (VA) mycorrhizal colonization of their host plants, and no investigation of those effects in soybean. In addition, there have been no investigations of the tripartite symbiotic association (VA mycorrhizal fungi, Bradyrhizobium japonicum and soybean) over a range of RZTs. A controlled-environment experiment was conducted to examine the effect of RZTs on VA mycorrhizal colonization, nutrient uptake, nodulation and nitrogen fixation, and plant growth and development. The experiment was organized as a randomized complete block split-plot design; the main-plot units consisted of four RZTs, 15, 18.2, 21.6 and 25°C; the four harvest stages and inoculation, or not, with mycorrhizal fungi formed the sub-plot units. The results of this study indicated that (1) the optimal RZT for mycorrhizal infection of Glomus versiforme was 21–22°C — above and below this range, mycorrhizal colonization was inhibited; (2) mycorrhizal colonization increased until flowering (the third harvest), but decreased thereafter at all tested RZTs; (3) mycorrhizal colonization had a negative effect on nodule establishment (nodule number) at lower RZTs, but a positive one at higher RZTs; and (4) the lower nodule number at lower RZTs was more than compensated for by increased mass per nodule so that mycorrhizal infection stimulated N2 fixation at lower RZTs.

Water and fertilizer nitrogen management to minimize nitrate pollution from a cropped soil in Southwestern Quebec, Canada

Nitrate-N (NO3--N) pollution of water resources is a widely recognized problem. Water and nitrogen fertilizer are the two most important factors affecting NO3--N movement to surface and groundwater. Field trials were conducted from 1998 to 2000 growing seasons to investigate the combined impacts of water table management (WTM) and N fertilization rate on NO3--N concentration in the soil profile and in drain discharge. There were two water table treatments: free drainage (FD) with open drains at a 1.0 m depth from the soil surface and subirrigation (SI) with a target water table depth of 0.6 m below the soil surface, and two N fertilizer rates: 120 kg N ha-1 (N120) and 200 kg N ha-1 (N200) in a split-plot design. Compared to FD, SI reducedNO3--N concentration in the soil by up to 50% averaged over the two N rates. Concentrations of NO3--N in drainage water fromSI plots were lower than those from FD by 55 to 73%. These findings suggest that SI can be used as a means of reducing soil NO3--N pollution and drainage water NO3--N concentrations.